Keywords: Multilayer Graphene Nanofilms (Nmag) Epitaxial Silicon (Epi-Si) Vertical Heterostructure Photodetectors Spectral Dependence
Note: This post uses the Sine Scientific Instruments OE1022 lock-in amplifier to make measurement.
[Overview]
In July 2024, Professor Yang Xu's team at the School of Integrated Circuits, Zhejiang University, published an article in Advanced Optical Materials entitled "Multilayer Graphene/Epitaxial Silicon Near-Infrared Self- Quenched Avalanche Photodetectors". Professor Yang Xu's team has investigated a novel multilayer graphene nanofilm (nMAG) and epi-Si vertical heterostructure photodetector (hereafter referred to as nMAG/epi-Si detector). The detector demonstrates high responsivity (2.51 mA-W-1) and detectivity (2.67 × 109 Jones) at 1550 nm, with low avalanche start-up voltage and self-extinguishing capability during avalanche multiplication, enabling real-time data transmission rates of up to 38 Mbps in the near-infrared optical communication data link.
In recent years, two-dimensional materials have received extensive attention in the field of optoelectronic devices due to their excellent optoelectronic properties. However, the low light absorption coefficient of these materials limits their detector applications. To overcome this problem, researchers have proposed to construct avalanche photodetectors (APDs) using 2D materials to achieve high gain through collisional ionisation. However, 2D materials introduce large noise while multiplying photogenerated carriers, which affects the detection of weak optical signals. In addition, 2D APDs usually require high bias voltage to generate collisional ionisation, resulting in high power consumption.
Based on the research of heterogeneous integration of 2D materials with silicon-based heterostructures, Xu Yang's group at the School of Integrated Circuits, Zhejiang University, fabricated a vertical heterostructured photodetector (nMAG/epi-Si detector) with low defect density and spectral dependence by heterogeneously integrating multilayer graphene with epi-Si. The multilayer graphene acts as the main light absorption layer to broaden the detection band of the silicon-based photodetector; the lightly doped epi-Si acts as the photogenerated electron multiplication region, effectively suppressing the generation and multiplication of hot carriers; and the heavily doped substrate silicon can form an ohmic contact between the silicon semiconductor and the metal electrode, improving the current transfer efficiency, thus reducing the overall power consumption and enhancing energy efficiency.
[Measurement method and some experimental results]
The research team prepared nMAG/epi-Si detectors by transferring high crystallinity nMAG as an absorber layer onto lightly doped epitaxial silicon (epi-Si) to form a vertical heterostructure (Fig. 1a).
In order to evaluate the performance of the nMAG/epi-Si detectors, the research team investigated the variation of four metrics, such as spectral response characteristics, external quantum efficiency (EQE), specific detectivity (D*) and noise equivalent power (NEP). By varying the wavelength of monochromatic light and measuring the short-circuit current of the detector at each wavelength, the team could obtain a spectral response curve reflecting the spectral response characteristics. Figure 1(b) demonstrates the spectral dependence of the nMAG/epi-Si detector. The spectral dependence was measured by a response measurement system equipped with a monochromator, a lock-in amplifier (Model OE1022, Scion Scientific Instruments), and a 150 W xenon lamp. nMAG/epi-Si detectors exhibit a spectrally dependent photoresponse in the range of 300-1100 nm, with a peak response of 0.38 A/W-1 . The dependence of the responsivity on the wavelength suggests that the generated photocurrents come from the epi-silicon. In addition the external quantum efficiency (EQE) of the nMAG/epi-Si detector is as high as 60% in the visible region, but decreases as the excitation wavelength increases in the near-infrared region. Figure 1(c) demonstrates the D* and NEP spectra of the nMAG/epi-Si detector in the wavelength range of 300-1100 nm. The results show that the D* and NEP can reach 6.63 × 1012 Jones and 5.80 × 10-13W/Hz-1/2, respectively.
Fig. 1 (a) Schematic of the nMAG/epi-Si photodetector, (b-c) spectral dependence of the nMAG/epi-Si photodetector
The Fig. 2 shows the test results under illumination from 532 nm to 1064 nm in the visible-near infrared band. Under reverse bias, the photocurrent increases with increasing power density, while the rectification ratio decreases. This is due to the fact that under reverse bias, the photogenerated carriers significantly change the concentration of minority carriers, which leads to the generation of photocurrent.
Fig. 2. Photovoltaic characteristics of nMAG/epi-Si detectors: I-V curves of nMAG/epi-Si detectors at (a) 532 nm and (c) 1064 nm at different power densities.
and photovoltaic properties of nMAG/epi-Si detectors at (a) 532 nm (b) and (c) 1064 nm (d) at different power densities.
The nMAG/epi-Si detector also exhibits self-quenching and high gain in avalanche mode, and can operate at 1550 nm. When the bias voltage is increased, the nMAG layer acts as an absorbing layer for the near-infrared long-wavelength spectrum in the nMAG/epi-Si device, as shown in Fig. 3(a). On the other hand, the lightly doped epi-Si layer is used as a multiplication layer, and by controlling the applied bias, the electric field in the nMAG layer can be adjusted in which light is absorbed and photocarriers are generated. Photogenerated electrons acquire large kinetic energy at large internal electric fields within a relatively wide depletion region and can be avalanche-multiplied by collisional ionisation with valence electrons in the lattice, which produces an exponential growth of free electrons leading to a rapid increase in photocurrent. At 1550 nm, the nMAG/epi-Si detector exhibits a high responsivity of 2.51 mA/W-1 and a detection rate of 2.67 × 109Jones, which is higher than that of the photodiode without avalanche (Fig. 4d).
Fig. 3. nMAG/epi-Si APD working mechanism and avalanche multiplication characteristics
The integration of the nMAG/epi-Si detector as an optical signal receiver in an optical communication system to measure eye charts further demonstrates its feasibility in practical optical applications, as shown in Fig. 4. nMAG/epi-Si APDs exhibit low noise currents, and their fast response capability enables them to reach a maximum real-time data rate of 38 Mbps when applied to near-infrared communication data links. in addition, the nMAG/epi-Si APDs can be used for near-infrared bicolour detection at room temperature. , the photodetector can also be used for near-infrared bicolour detection at room temperature and has been successfully experimented.
Fig. 4. Performance characterisation of NIR optical communication
[Summary]
The research team fabricated an nMAG/epi-Si detector with low defect density and spectral dependence, which has high responsivity and detectivity at 1550 nm, suitable for near-infrared optical communications and high-resolution imaging. It has high gain and self-quenching characteristics at low reverse bias, reducing noise and improving signal stability. In addition, the detector's low noise and fast response enable it to achieve a data rate of 38 Mbps. These properties indicate that the nMAG/epi-Si heterostructure has great potential in the field of infrared detection and optical communication.
[Reference]
